Instead of asking, "Would you like fries with that?" Schwartz says, fast-food restaurants could make a dent in Americans' waistlines if they asked, "How many fries would you like with that?"

Given the choice, many Americans would opt for smaller portions of starchy side dishes, Schwartz and her co-authors conclude in their new study.

Posting calories in fast-food and chain restaurants has had little or no impact on calorie consumption to date, the authors write. And, Schwartz says, although healthier meals test well, relatively few customers order them.

So, at a Chinese fast-food restaurant on the Duke University campus in Durham, N.C., they field-tested a different approach: Have servers ask customers if they want to downsize that side of steamed rice, fried rice, or chow mein.

A standard portion of the starchy side dishes was 400 calories; the reduced portion was half that.

"Servers at this particular restaurant had to be retrained in order to do the serving this way," Schwartz says. "They looked at us like we were crazy." The restaurant shall remain nameless, she says, but she adds that you can find it at any airport food court.

Customers rarely took the initiative to ask for smaller portions of the side dishes, the researchers found. But when servers offered the option, 14% to 33% of the customers took them up on it, whether or not they were given a $.25 discount. Ironically, the scientists found that customers were less likely to accept the offer of a smaller portion when calorie counts were posted than when they weren't.

Clean-Plate Club

Diners who stuck with the regular-sized side dish were just as likely to clean their plate as those who opted for the smaller side. "A lot of people say, 'Well, it's here. I might as well just go ahead and finish it,'" Schwartz says.

But if restaurants gave customers the choice of a smaller portion of side dishes such as rice or fries, she says, they could end up cutting costs as well as calories. "It's a win-win for everybody," she says.

Eric Finkelstein, PhD, deputy director of the Health Services and Systems Research Program at Duke-NUS Graduate Medical School in Singapore, questioned whether customers would continue to choose smaller portions.

"The big problem with the study is customers were only asked to do it once," Finkelstein tells WebMD in an email. He co-authored a study published last year that found posting calories at certain chain restaurants in the Seattle area had no impact on customers' food purchases.

"Asking about smaller portions is fine but unlikely to be of more than academic interest," Finkelstein says. "After all, there is a reason that chains offer the size portions that they do. It is profit-maximizing, or they would not be doing it. Over time, many customers may realize that they are hungry after the meal and would go back to their larger portions."

Schwartz emphasized that only the side dish was downsized, or, as she prefers to call it, rightsized. Customers, who were not aware they were being observed, still received a heaping portion of their entrée of choice. "If you saw this food," she says, "there's no way you could complain you're still hungry."

Schwartz and her co-authors published their findings in the latest issue of Health Affairs.